Thermal diffusion apparatus



Oct. 18, 1955 JONES 2,720,976

THERMAL DIFFUSION APPARATUS Filed Feb. 27, 1952 SEPARATION An x I0INVENTOR.

FEED RATE, .h.

ARTHUR LETCHER JONES ATTORNEYS.

United States Patent Ofifice 2,720,976 Patented Oct. 18, 1955 THERMALDIFFUSIQN APPARATUS Arthur Letcher Jones, Lyndhurst, Ohio, assignor toThe Standard Oil Company, Cleveland, Ohio, a corporation of OhioApplication February 27, 1952, Serial No. 273,738 4 Claims. (Cl.210-525) The present invention relates to a new and useful improvementin apparatus for separating liquid mixtures by continuous thermaldiffusion.

It has been known for some time that it is possible to separate liquidmixtures into two or more dissimilar fractions by subjecting a thin filmof liquid mixture to a temperature gradient. For many years this wasaccomplished in apparatus consisting essentially of two closely spaced,parallel walls forming a narrow slit and provided at each end of theslit with a reservoir having a volume considerably in excess of thevolume of the slit. By filling such an apparatus with a liquid mixtureand maintaining the opposed walls at ditferent temperatures, the liquidin one reservoir becomes enriched in one component of the liquidmixture, and the liquid in the other reservoir becomes impoverished insaid component or enriched in another component.

After many hours or several days, the liquids in the two reservoirs areseparately withdrawn.

The amount of liquid that can be subjected to thermal diffusion inapparatus of this type is extremely small, being measured in tens ofccs., and the time and heat energy required is so great that thetechnique of separating liquid mixtures by thermal diffusion remainednothing more than a laboratory curiosity for some eighty years.

More recently it has been proposed, e. g., in Patents 2,541,069-071, tosubject liquid mixtures to thermal diffusion by maintaining atemperature gradient across a narrow slit formed by closely spacedconcentric or fiat walls, and continuously passing a stream of theliquid mixture through the slit. It was found that by such means therate of separation obtainable is much improved.

The slit widths of liquid thermal diffusion columns are extremelynarrow, i. e., less than about 0.15 and desirably not more than about0.06". When continuous liquid thermal diffusion is carried out inapparatus designed for concurrent flow, i. e., where the liquid mixtureto be subjected to thermal diffusion is introduced at one portion of avertical, horizontal or inclined slit, and the dissimilar fractions arewithdrawn from adjacent the hot and cold walls, respectively, at pointsremote from the point of introduction, there is, theoretically, no limitto the minimum slit width due to the absence of endwise thermalcirculation within the slit. For practical considerations, however, theminimum slit width for concurrent flow is of the order of about 0.01".

There is reason to believe that the withdrawal of dissimilar fractionsof a liquid mixture from a thermal diffusion column, particularly whenthey move through the slit concurrently, causes sufficient turbulence tobring about a rather considerable remixing, immediately prior toWithdrawal, of the fractions separated within the slit. The end resultof this is that the degree of separation obtainable is less than thedegree of separation actually obtained within the slit, i. e., theefiiciency of withdrawal is not commensurate with the efficiency ofseparation by thermal diffusion.

The present invention is addressed particularly to improving theefiiciency with which fractions separated by thermal diffusion within acolumn can be withdrawn. In essence, the improvement with which thepresent invention is concerned is an improved form of withdrawal port,which comprises an elongated section in the face of one of the wallsthat is porous and has a longitudinal axis transversely of the directionof flow of liquid within the slit, and a passage for liquid extending tothe exterior of the apparatus, one portion of which is substantiallycoextensive with and contiguous to the porous section.

The porous section may be an integral portion of one of the walls orbuilt up by providing an elongated groove in the face of one of thewalls forming the thermal diffusion slit and fitting an elongated stn'pmember of porous, inert material in the groove, one face of the stripmember being in substantially the same plane as the wall face.

The strip member may be of any suitable material or structure capable ofdistributing substantially uniformly the flow of liquid through it overthe entire area of the member. Thus, for example, it may be, andpreferably is in the form of a strip of sintered metal or porous ceramicmaterial, e. g., porcelain, or it may have a composite structure, e. g.,it may comprise fine screening material at the surfaces enclosingfibrous packing material, clay or the like.

To further promote substantially uniform distribution of flow of liquidthrough the porous section or strip member, the portion of the passagethat is substantially coextensive with and contiguous thereto and theportion of the passage extending to the exterior of the apparatus aresufiiciently large in volumetric proportions in relation to volumetricflow capacity of the porous strip member to assure substantially uniformflow through all of the pores but not so large as to delay unduly thedisplacement from the passage of unseparated liquid mixture with whichit is initially filled upon commencement of the liquid thermal diffusionprocess, or hold up more than necessary of the separated fraction duringthe continued operation. The passage, particularly the portioncontiguous to the porous section, may be tapered to assist in uniformwithdrawal, especially if the volumetric proportions are relativelysmall.

One of the primary advantages of the withdrawal port of this inventionis that it increases enormously the chiciency, probably due to aminimization of turbulence, with which fractions separated within athermal diffusion slit can actually be removed from the slit.

This and other advantages, as well as the utility of the invention, willbecome further apparent from the following detailed description madewith reference to the accompanying drawing, wherein:

Figure l is a cross-sectional view of a liquid thermal diffusionapparatus, provided with two withdrawal ports illustrative of theinvention;

Figure 2 is a cross-sectional line 22 of Figure l; and

Figure 3 is a graph demonstrating the superior efficiency of withdrawalports constructed in accordance with this invention as compared withthat of a groove type take-off port.

Referring now to Figures 1 and 2 illustrating schematically a liquidthermal diffusion apparatus utilizing two Withdrawal ports constructedin accordance with the present invention, the opposed walls 10 and 11having wall faces 12 and 14, respectively, are substantially parallel,liquid impervious and stationary. The wall faces 12 and 14 are separatedfrom one another a distance of less than view taken along section about0.15", desirably not more than 0.06" and preferably from about 0.01 toabout 0.035" apart by one or more gaskets 16 to form a narrow slit 17.The walls and Y11 may be maintained in their ,position relative of thewall 10 and a'passage through which the liquid mixture :can'beintroduced from outside the apparatus by any suitable means and to theslit 17 by way of groove 19.

The apparatus ures 1 and 2 is provided, at the end of the slit 17-remote from the inlet port 19, 20, with two withdrawal ports, oneiinwall 10 and the other in wall 11. The withdrawal port in wall 10, forexample, comprises an elongated groove in the face12 and an elongatedstrip member 21 of porous material fitted in the groove. The face 22 ofthe strip member 21 is in substantially the same plane as 1 k the wallface 12. Under, adjacent and substantially coextensive With the str'mmember 21 there is an elongated flow-equalizing portion 24 of a passagefor liquid that communicates with a passage portion 26 extending to theexterior of the wall 10.

A series of 'testswere run on apparatus essentially similar to thatillustrated .in Figures 1 and 2 wherein the Width of the slit 17 was0.035", the slit was in the vertical position shown, and the effectivelength and breadth of the slit were both 10'. One wall was maintained ata temperature of 270 F. and the other was maintained at a temperature of70F. In one series of tests the two withdrawal ports were constructed asshown in the drawing, the strip member 21 being a porous, sintered metalstrip.

In a comparative series of test runs all conditions were identical,except that instead of the withdrawal ports illustrated in Figures 1 and2, grooves having a depth of oneeig'hth of an inch and a width ofbetween two and three sixteenths of .an inch, connected by means ofequally spaced holes in the bottom of the groove to a passage similar topassage 26 of the apparatus illustrated, but extending the entire lengthof the groove, were used. I

In each test a 50/50 mixture of cetane and methylnaphthalene was fedinto the column by way of inlet port 19, 20 at various rates of flow,and fractions from adjacent the hot and cold walls, respectively, werewith- I drawn through the oppositely disposed withdrawal ports at equalrates. To measure the degree of 'separationobtained'in 'each instance,the index of refraction at C.

of the two fractions were obtained, the difference in indices ofrefraction between the hot wall product and the cold wall product beingan accurate indicator of the extent to which separation took place. Theresults of these tests are illustrated "-in the graph of Figure 3,wherein the curve A represents the degree of separation, in terms ofdifierence between-the indices of refraction of the hot wall and coldwall products obtained with the thermal diffusion column having thewithdrawal ports of this invention, andcurve B represents the degree ofseparation, expressed similarly, obtained under identical conditions butwith asimple groove type of withdrawal port. These curves reveal thatwith the withdrawal ports of the present invention, the degree ofseparation rises rapidly as the feed rate is increased to about oneliter per hour and remains practically level at higher feed rates,whereas with the simple groove type withdrawal ports the degree ofseparation is less than one-fifth as great at feed rates ranging fromabout 1 to 6 liters per hour.

'The degree of separation that takes place within a thermal diffusionslit is determined, for any given liquid mixture, by such conditions astemperature gradient, slit width, wall surface area and residence timeof the liquid, all of which were the same in the comparative tests. Itisself-evident', therefore, that the results obtained in theseillustrated by way of example in Fig.

tests are indicative of the ,great superiority of the withdrawal portsof this invention over the groove type of withdrawal port. Thewithdrawal ports of this invention make it possible to separatephysically the fractions concentrated adjacent the hot and cold wallswithin the thermal diffusion slit with far greater efficiency than hasheretofore been possible.

It is to be understood that various changes and modifications willreadily occur to those skilled in the art upon reading this description.All such changes and modifications are intended to be included withinthe scope of the invention as defined in the appended claims.

I claim:

1. Liquid thermal diffusion apparatus comprising two tially uniformnarrow slit, means for relatively heating and cooling said walls toimpose a temperature gradient across said slit an inlet communicatingwith the slit and at least two outlets communicating with the slit,wherein at least one of the outlets comprises an elongated poroussection in the face of one of the walls, one face of the porous sectionbeing in substantially the same plane as the wall face, and a passagefor liquid extending to the exterior of the apparatus, one portion ofsaid passage being substantially coextensive with and contiguous to theporous section.

2. The apparatus defined in claim 1 wherein the slit forming walls arevertical and two outlets are disposed, one in each wall, at one end ofthe slit.

3. Liquid thermal diffusion apparatus comprising two substantiallyparallel, liquid-impervious stationary walls, the opposed faces of whichare smooth and spaced apart a distance less than about 0.15 inch to forma substantially uniform narrow slit, means for relatively heating andcooling said walls to impose a temperature gradient across said slit aninlet communicating with the slit and at least two outlets communicatingwith the slit, wherein at least one of the outlets comprises anelongated groove in the face of one of the walls; an elongated stripmember of porous material fitted in the groove, one face of the stripmember being in substantially the same plane as the wall face; and apassage for liquid extending to the exterior of the apparatus, oneportion of said passage being substantialy' coextensive with andcontiguous to the strip member.

4. Liquid thermal difiusion apparatus comprising two substantiallyparallel, liquid-impervious stationary walls, the opposed faces of whichare smooth and spaced apart a distance less than about 0.15" to form asubstantially uniform narrow slit, means for relatively heating andcooling said walls to impose a temperature gradient across said slitwherein at least one of the walls is provided with an elongated poroussection in the face there of, said elongated porous section having itsgreatest dimension extending in a direction transverse to the direc--tion of fiow of liquid within'the slit and one face of the poroussection being in substantially the same plane as the wall face, and witha passage for liquid extending to the exterior of the apparatus, oneportion of said passage being substantially coextensive with andcontiguous to the porous section, and at least one of said walls havingan inlet for supplying liquid to said slit, said inlet being spaced fromsaid porous section.

References Cited in the file of this patent UNITED STATES PATENTS

1. LIQUID THERMAL DIFFUSION APPARATUS COMPRISING TWO SUBSTANTIALLYPARALLEL, LIQUID-IMPERVIOUS STATIONARY WALLS, THE OPPOSED FACES OF WHICHARE SMOOTH AND SPACED APART A DISTANCE LESS THAN ABOUT 0.15 INCH TO FORMA SUBSTAN TIALLY UNIFORM NARROW SLIT, MEANS FOR RELATIVELY HEATING ANDCOOLING SAID WALLS TO IMPOSE A TEMPERATURE GRADIENT ACROSS SAID SLIT ANINLET COMMUNICATING WITH THE SLIT AND AT LEAST TWO OUTLETS COMMUNICATINGWITH THE SLIT, WHEREIN AT LEAST ONE OF THE OUTLETS COMPRISES ANELONGATED POROUS SECTION IN THE FACE OF ONE OF THE WALLS, ONE FACE OFTHE POROUS SECTION BEING IN SUBSTANTIALLY THE SAME PLANE AS THE WALLFACE, AND A PASSAGE FOR LIQUID EXTENDING TO THE EXTERIOR OF THEAPPARATUS, ONE PORTION OF SAID PASSAGE BEING SUBSTANTIALLY COEXTENSIVEWITH AN CONTIGUOUS TO THE POROUS SECTION.